Embodiments of the present disclosure relate generally to a magnetic component having at least one graded region and a method of forming the same. More particularly, the disclosure relates to a magnetic component including at least one region with a graded nitrogen content, and formation of the same by nitriding.
The need for high power density and high efficiency electrical machines, such as, for example, electric motors and generators has long been prevalent for a variety of applications, particularly for hybrid and/or electric vehicle traction applications. The current trend in hybrid/electric vehicle traction motor applications is to increase rotational speeds to increase the machine's power density, and hence reduce its mass and cost. However, it is recognized that when electrical machines are used for traction applications in hybrid/electric vehicles, there is a tradeoff between power density, efficiency, and the machine's constant power speed range. This tradeoff may present numerous design challenges.
Specifically, magnetic materials used in rotating electric machines generally serve multiple functions. Efficient coupling of magnetic fluxes to complementary poles across an air gap in the electric machines are desirable. Soft magnetic materials of the magnetic components may also bear a significant fraction of the mechanical and structural loads of the electric machine. Sometimes there may be tradeoffs between efficient magnetic utilization of the material and mechanical load bearing capability of the magnetic materials. Alternatively, sometimes speed rating of a machine may be lowered in order to allow a machine topology with efficient magnetic utilization. Therefore, it is desirable to have a material that can avoid the trade-offs between efficiency and operability of the electric machines by locally controlling the magnitude of the saturation magnetization of the soft magnetic material used in electric machines.
The power density of an electric machine may be increased by increasing the machine size, improving thermal management, increasing rotor speed, or by increasing the magnetic utilization. The magnetic utilization may be increased by using a combination of processing and alloying of a rotor lamination to create a multi-phase magnetic material by developing localized areas of high and low permeability. The localized areas of high and low permeability generally reduce flux losses during rotor operation. An abrupt change of saturation magnetization in the localized areas of high and low permeability of the machine may result in undesirable torque ripple in the electric machine.
A range of ferrous based soft magnetic compositions of the rotor lamination may be austenitized by a combination of processes to form regions of low permeability. This phase transformation at selected regions is normally thermally driven in the presence of carbides in the alloy. Upon local heating, the carbides that are present at selected locations dissolve in the matrix and depress the martensite start temperature, thereby aiding the stabilization of austenite regions at room temperature. However, the presence of carbides in a magnetic microstructure is known to increase coercivity and to lower the magnetic saturation, as compared to traditional ferrous-based magnetic steels. A different method of controlling the stabilization of the austenite phase at room temperature in different regions of the soft magnet, while starting from a substantially single phase microstructure, is desired to reduce torque ripple for a smooth motor operation. Embodiments of the present disclosure address these and other needs.